Salmonella has an average genomic GC content of 50-52%, which is similar to that of Escherichia coli and Shigella. The genus Salmonella is a pathogenic microorganism that causes infections in livestock as well as in humans. Serological division has it that Salmonella enterica, a species of Salmonella bacterium, has a variety of serovars including Salmonella gallinarum, Salmonella pullorum, Salmonella typhimurium (ST), Salmonella enteritidis (SE), Salmonella typhi, Salmonella choleraesuis (SC), Salmonella derby (SD). Of them, choleraesuis and derby are swine-adapted pathogens, gallinarum and pullorum are fowl-adapted pathogens, typhimurium and enteritis are pathogenic for humans and animals, and typhi is a human-adapted pathogen, all of which cause illness in their respective species, resulting in tremendous damage to farmers and consumers (Zoobises Report; United Kingdom 2003).
Recently, implementation of HACCP (Hazard analysis and critical control points) has become a mandatory requirement for all slaughterhouses in Korea as of Jul. 1, 2003, because of the high risk of contamination in the course of manufacturing livestock products with salmonella, which causes direct damage to pigs, and meat hygiene and is found in the digestive tract of pigs (Jae-gil Yeh. Characterization and Counterplan of Salmonellosis in Pigs. Monthly Magazine of Pig Husbandry. 2004).
Paratyphoid, the acute or chronic infectious disease in the digestive tract of pig caused by salmonella infection, is characterized by gastroenteritis and septicaemia and mainly occurs during the fatting period. In particular, some of pathogenic bacteria that cause this disease can cause food poisoning in humans through meat ingestion, and thus it is a disease having major public health importance. A variety of types of salmonella bacteria can be pathogenic. Among them, Salmonella choleraesuis and Salmonella typhisuis known to cause hog cholera are the major causes of acute salmonella septicaemia. Acute enteritis occurs during the fattening period, and is accompanied by irregular appetite, severe watery diarrhea, high fever, loss of vitality, pneumonia, and nervous signs. Discoloration of the skin may occur in some severe cases. Salmonella typhimurium, Salmonella enteritidis, and Salmonella derby are the major causes of chronic enteritis.
Salmonellosis is caused by oral route through feed or water contaminated with salmonella, and thus these routes should be prevented. Contaminated feed, raw materials or water, or adult pigs carrying the pathogen can be major sources of infection. During the acute period of infection, pigs shed up to 106 Salmonella choleraesuis or 107 Salmonella typhimurium per gram of feces. However, many experimental infections reported successful disease reproduction with a dose of 108 to 1011 Salmonella. In an experiment injecting 103 Salmonella into pigs, the injected pigs showed no symptoms of the disease, but other pigs raised in the same pen showed typical clinical symptoms. These results indicate that a large amount of salmonella grow in naturally infected pigs, resulting in the infection of other pigs (Jung-Bok Lee. Control of the Recent Outbreaks of Porcine Salmonella and Proliferative Enteropathy. Korea Swine Association. 2009).
At present, severe viral infections such as Porcine Reproductive and Respiratory Syndrome (PRRS) and Porcine Circovirus (PCV2) have been causing tremendous economic losses to the swine industry in Korea, and thus disease management has been focused on these diseases. Since these bacterial diseases may cause tremendous damage comparable to that caused by viral diseases beginning with a ban on the use of in-feed antibiotics and an investigation of disease occurrence or disease management should be performed in advance (Jung-Bok Lee. Control of the Recent Outbreaks of Porcine Salmonella and Proliferative Enteropathy, Infectious Disease Laboratory, College of Veterinary Medicine, Konkuk University, Livestock Product Safety, 2010) (Robert W. Wills, Veterinary Microbiology, 1999). Meanwhile, bacteriophage, also called phage, is a specialized type of virus that infects only particular bacteria and controls the growth of bacteria, and can self-replicate only inside the host bacteria. After the discovery of bacteriophages, a great deal of faith was initially placed in their use for infectious-disease therapy. However, when broad spectrum antibiotics came into common use, bacteriophages were seen as unnecessary due to a specific target spectrum. Antibiotics or antimicrobial agents have been widely used for the treatment of infectious diseases caused by bacterial infection. Nevertheless, the misuse and overuse of antibiotics resulted in rising concerns about antibiotic resistance and the harmful effects of residual antibiotics in foods. However, the removal of current in-feed antibiotics might increase occurrence of bacterial diseases including salmonellosis that have been controlled by antibiotics, as expected in the experiment data or from other countries. Thus, there is an urgent need to establish a detailed practical guideline for salmonella management (Jung-Bok Lee. Control of the Recent Outbreaks of Porcine Salmonella and Proliferative Enteropathy, Infectious Disease Laboratory, College of Veterinary Medicine, Konkuk University, Livestock Product Safety. 2010).
These growing concerns have led to a resurgence of interest in bacteriophage. Seven bacteriophages for control of E. coli O157:H7 are disclosed in U.S. Pat. No. 6,485,902 (2002) and two bacteriophages for control of various microorganisms are disclosed in U.S. Pat. No. 6,942,858 (issued to Nymox in 2005). Many companies have been actively trying to develop various products using bacteriophages. EBI food system (Europe) developed a food additive for preventing food poisoning caused by Listeria monocytogenes, named Listerix-P100, which is the first bacteriophage product approved by the USFDA. A phage-based product, LMP-102 was also developed as a food additive against Listeria monocytogenes, approved as GRAS (Generally Regarded As Safe). In 2007, a phage-based wash produced by OmniLytics was developed to prevent E. coli 0157 contamination of beef during slaughter, approved by USDA's Food Safety and Inspection Service (FSIS). In Europe, Clostridium sporogenes phage NCIMB 30008 and Clostridium tyrobutiricum phage NCIMB 30008 were registered as feed preservative against Clostridium contamination of feed in 2003 and 2005, respectively. Such studies show that research into bacteriophages for the control of antibiotic-unsusceptible bacteria and contamination of livestock products by zoonotic pathogens is presently ongoing.
However, most of the phage biocontrol studies have focused on the control of E. coli, Listeria, and Clostridium. Salmonella is also a zoonotic pathogen, and damages due to this pathogen are not reduced. As mentioned above, since Salmonella exhibits multiple drug resistances, nationwide antimicrobial resistance surveillance has been conducted in Korea under the Enforcement Decree of the Act on the Prevention of Contagious Disease (Executive Order 16961), Enforcement Ordinance of the Act on the Prevention of Contagious Disease (Ministry of Health and Welfare's Order 179), and Organization of the National Institute of Health (Executive Order 17164). Accordingly, there is a need for the development of bacteriophages to control Salmonella. 